Deposition of carbon inside gaps of castellated tungsten blocks with intrinsic misalignment

Highlight• The castellated blocks are manufactured and installed at central divertor in KSTAR. • The surface density of inside the toroidal and poloidal gap of castellated blocks is in a range from 0.5×10¹⁵ up to 6.7×10¹⁵ atom/cm². • Compare the carbon deposition on the toroidal and poloidal gaps, contribution of each species can be separated. • The contribution of neutrals is common for both gaps. • The carbon density on chamfered shape is the lowest in to the four shapes. • At the Raman spectra, decrease of I(D)/I(G) as a function of distance from the gap entrance indicates the increase of hydrogen contents. When PFCs have castellated structure, co-deposition of fuel inside gaps between castellated blocks is an important issue. Four different shapes of castellated tungsten blocks were fabricated to study corresponding issues in KSTAR: Conventional "basic" rectangular shape, single chamfer leading edge, double-chamfer and rounded edge, with two different poloidal gap distances of 0.5 mm and 1.0 mm. These tungsten blocks were exposed plasma of L- and H-mode discharges during a whole campaign in 2014. The blocks were taken out from the vacuum vessel after the campaign. Gap deposition was analyzed by Electron Probe X-ray Micro Analyzer (EPMA) to obtain carbon surface density (atoms/cm²), and by Raman spectroscopy to identify chemical bonding structure of carbon deposits in gaps. The carbon surface density in toroidal and poloidal gaps was in a range from 0.5 × 10¹⁵ atom/cm² up to 6.7 × 10¹⁵ atom/cm². At the gap entrance, contribution of ions is 6.0–6.7 × 10¹⁵ atom/cm², decreased down to 1.0 × 10¹⁵ atom/cm² at a depth of 0.5 mm, and remains constant afterwards. The contribution of charge exchange neutral is about 3.0 × 10¹⁵ atom/cm² at the gap entrance, and then gradually decreases as a function of distance from the entrance. Deposition in 1.0 mm wide gaps show much larger deposition patterns and that particles have reached much deeper inside the gap. Raman spectra show that the intensity ratio I(D)/I(G) decreases from top to the depth of 5 mm indicating the increase of hydrogen contents.